It is known that the compression of an internal combustion engine can be measured by measuring the current drawn by the electric starter motor while cranking the engine with ignition inhibited. See U.S. Pat. NO. 3,839,906 issued to R. E. Hanson on Oct. 8, 1974, on an "Apparatus for Engine Compression Testing", and U.S. Pat. No. 3,765,233 issued to R. Germann on Oct. 16, 1973, on a "Device For Measuring Compression of Piston Engines." The motor current fluctuates with maximum values corresponding in time with compression strokes in the engine. The above first-mentioned patented apparatus (1) compares current peaks to determine whether some cylinders have significantly less compression than other cylinders, and, if the cylinders are substantially equal in compression, (2) measures the highest current peak relative to the current average to determine whether all cylinders have less than a satisfactory level of compression. If the first test is failed, repairs to parts of or associated with one or more individual cylinders are needed. If the second test is failed, repairs to all cylinders or parts shared by all cylinders are required.
In the second test of compression, where there is little difference between individual cylinders, the current peak corresponding with the cylinder having the highest compression is measured relative to the current average corresponding with the average compression in all cylinders. The difference between the highest peak current and the average current is substantially the same as the difference between the average of the individual current peaks and the average current.
This second test of compression compares the peak-to-average current with a known value of peak-to-average current measured while cranking an engine known to have good compression at a given (sea level) ambient pressure, and at a throttle position insuring a negligible intake manifold vacuum relative to the ambient pressure. If the test of an engine is actually performed under conditions of lower ambient (atmospheric) pressure and higher intake manifold vacuum, the peak-to-average current may be up to about 30 percent lower than the peak-to-average current would be at sea level with open throttle. A low absolute pressure at the intake manifold results in less air in the cylinder to be compressed by the piston, and consequently less resistance to the starter motor, and less starter motor current.
The significance of these pressure variations can easily be shown by example. For a vehicle at sea level with a wide open throttle, the absolute intake manfiold pressure is close to 14.7 PSIA. For the same vehicle tested at high altitude (Denver, Colorado, for example) with a wide open throttle, the absolute intake manifold pressure would have dropped to about 12.2 PSIA and if the throttle were not wide open the pressure drop through the carburetor could easily drop this to about 10.0 PSIA. Thus, variations in engine test conditions, test location, and even weather could change the engine compression pressure by as much as 30%. Therefore, the compression test should take existing intake manifold pressure into account.